A lithographic apparatus is an apparatus that can be used in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred by a radiation beam via a projection system onto a target portion on a substrate, such as a silicon wafer. Transfer of the pattern is typically performed via imaging onto a layer of radiation-sensitive material provided on the substrate. The location on which the radiation beam is incident on the target portion is referred to as the exposure location.
The cross-section of the radiation beam is typically much smaller than the surface of the substrate. So in order to expose all the target portions on the surface of the substrate, the substrate is moved relative to the projection system. The lithographic apparatus has a stage system to move the substrate relative to the projection system. The stage system is able to move the substrate in such a way that the target portions are placed consecutively in the path of the radiation beam.
To place each of the target portions in the path of the radiation beam correctly, the lithographic apparatus is provided with a position measurement system. The position measurement system measures a position of the stage system. The position measurement system comprises an encoder and a scale. The scale has a grating surface. The encoder is arranged to receive radiation beams from the grating surface of the scale. The encoder is able to detect a change in a phase of the radiation beams when the grating surface moves relative to the encoder. In dependence of the phase, the encoder is able to generate a position signal representative of the position of the grating surface relative to the encoder.